Magnetic and magneto-optical (MO) recording media are widely used in various applications, e.g., in hard disk form, particularly in the computer industry, for storage and retrieval of large amounts of data/information. Typically such media require pattern formation in the major surface(s) thereof for facilitating operation, e.g., servo pattern formation for enabling positioning of the read/write transducer head over a particular data band or region.
Magnetic and magneto-optical (MO) recording media are conventionally fabricated in thin film form; the former are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation (i.e., parallel or perpendicular) of the magnetic domains of the grains of the magnetic material constituting the active magnetic recording layer, relative to the surface of the layer.
In operation of magnetic media, the magnetic layer is locally magnetized by a write transducer or write head to record and store data/information. The write transducer creates a highly concentrated magnetic field which alternates direction based on the bits of information being stored. When the local magnetic field applied by the write transducer is greater than the coercivity of the recording medium layer, then the grains of the polycrystalline magnetic layer at that location are magnetized. The grains retain their magnetization after the magnetic field applied by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The pattern of magnetization of the recording medium can subsequently produce an electrical response in a read transducer, allowing the stored medium to be read.
A typical contact start/stop (CSS) method employed during use of disk-shaped recording media, such as the above-described thin-film magnetic recording media, involves a floating transducer head gliding at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between mutually sliding surfaces of the transducer head and the disk. During reading and recording (writing) operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the transducer head is freely movable in both the circumferential and radial directions, thereby allowing data to be recorded and retrieved from the disk at a desired position in a data zone.
Adverting to FIG. 1, shown therein, in simplified, schematic plan view, is a magnetic recording disk 30 (of either longitudinal or perpendicular type) having a data zone 34 including a plurality of servo tracks, and a contact start/stop (CSS) zone 32. A servo pattern 40 is formed within the data zone 34, and includes a number of data track zones 38 separated by servo tracking zones 36. The data storage function of disk 30 is confined to the data track zones 38, while servo tracking zones 36 provide information to the disk drive which allows a read/write head to maintain alignment on the individual, tightly-spaced data tracks.
Although only a relatively few of the servo tracking zones are shown in FIG. 1 for illustrative simplicity, it should be recognized that the track patterns of the media contemplated herein may include several hundreds of servo zones to improve head tracking during each rotation of the disk. In addition, the servo tracking zones need not be straight radial zones as shown in the figure, but may instead comprise arcs, intermittent zones, or irregularly-shaped zones separating individual data tracks.
In conventional hard disk drives, data is stored in terms of bits along the data tracks. In operation, the disk is rotated at a relatively high speed, and the magnetic head assembly is mounted on the end of a support or actuator arm, which radially positions the head on the disk surface. If the actuator arm is held stationary, the magnetic head assembly will pass over a circular path on the disk, i.e., over a data track, and information can be read from or written to that track. Each concentric track has a unique radius, and reading and writing information from or to a specific track requires the magnetic head to be located above that track. By moving the actuator arm, the magnetic head assembly is moved radially on the disk surface between tracks. Many actuator arms are rotatable, wherein the magnetic head assembly is moved between tracks by activating a servomotor which pivots the actuator arm about an axis of rotation. Alternatively, a linear actuator may be used to move a magnetic head assembly radially inwardly or outwardly along a straight line.
As has been stated above, to record information on the disk, the transducer creates and applies a highly concentrated magnetic field in close proximity to the magnetic recording medium. During writing, the strength of the concentrated magnetic field directly under the write transducer is greater than the coercivity of the recording medium, and grains of the recording medium at that location are magnetized in a direction which matches the direction of the applied magnetic field. The grains of the recording medium retain their magnetization after the magnetic field is removed. As the disk rotates, the direction of the writing magnetic field is alternated, based on bits of the information being stored, thereby recording a magnetic pattern on the track directly under the write transducer.
On each track, eight “bits” typically form one “byte” and bytes of data are grouped as sectors. Reading or writing a sector requires knowledge of the physical location of the data in the data zone so that the servo-controller of the disk drive can accurately position the read/write head in the correct location at the correct time. Most disk drives use disks with embedded “servo patterns” of magnetically readable information. The servo patterns are read by the magnetic head assembly to inform the disk drive of track location. In conventional disk drives, tracks typically include both data sectors and servo patterns and each servo pattern typically includes radial indexing information, as well as a “servo burst”. A servo burst is a centering pattern to precisely position the head over the center of the track. Because of the locational precision needed, writing of servo patterns requires expensive servo-pattern writing equipment and is a time consuming process.
Commonly assigned, U.S. patent application Ser. No. 10/082,178, filed Feb. 26, 2002 now abandonment Aug. 23, 2005, the entire disclosure of which is incorporated herein by reference, discloses a method and apparatus for reliably, rapidly, and cost-effectively forming very sharply defined magnetic transition patterns in a magnetic medium containing a longitudinal or perpendicular type magnetic recording layer without requiring expensive, complicated servo writing equipment/techniques incurring long processing intervals.
Specifically, the invention disclosed in U.S. patent application Ser. No. 10/082,178 is based upon recognition that a stamper/imprinter comprised of a magnetic material having a high saturation magnetization, Bsat, i.e., Bsat≧about 0.5 Tesla, and a high permeability, μ i.e., μ≧ about 5, e.g., selected from Ni, NiFe, CoNiFe, CoSiFe, CoFe, and CoFeV, can be effectively utilized as a contact “stamper/imprinter” for contact “imprinting” of a magnetic transition pattern, e.g., a servo pattern, in the surface of a magnetic recording layer of a magnetic medium (“workpiece”), whether of longitudinal or perpendicular type. A key feature of this invention is the use of a stamper/imprinter having an imprinting surface including a topographical pattern, i.e., comprised of projections and depressions corresponding to a desired magnetic transition pattern, e.g., a servo pattern, to be formed in the magnetic recording layer. An advantage afforded by the invention is the ability to fabricate the topographically patterned imprinting surface of the stamper/imprinter, as well as the substrate or body therefor, of a single material, as by use of well-known and economical electro-forming techniques.
According to the invention, the magnetic domains of the magnetic recording layer of the workpiece are first unidirectionally aligned (i.e., “erased” or “initialized”), as by application of a first external, unidirectional magnetic field Hinitial of first direction and high strength greater than the saturation field of the magnetic recording layer, typically ≧2,000 and up to about 20,000 Oe. The imprinting surface of the stamper/imprinter is then brought into intimate (i.e., touching) contact with the surface of the magnetic recording layer. With the assistance of a second externally applied magnetic field of second, opposite direction and lower but appropriate strength Hre-align, determined primarily by the coercivity of the media to be patterned and in part by Bsat/μ of the stamper material (typically ≧100 Oe, e.g., from about 2,000 to about 4,500 Oe), the alignment of the magnetic domains at the areas of contact between the projections of the imprinting surface of the stamper/imprinter (in the case of perpendicular recording media, as schematically illustrated in FIG. 2) or at the areas facing the depressions of the imprinting surface of the stamper/imprinter (in the case of longitudinal recording media, as schematically illustrated in FIG. 3) and the magnetic recording layer of the workpiece is selectively reversed, while the alignment of the magnetic domains at the non-contacting areas (defined by the depressions in the imprinting surface of the stamper/imprinter) or at the contacting areas, respectively, is unaffected, whereby a sharply defined magnetic transition pattern is created within the magnetic recording layer of the workpiece to be patterned which essentially mimics the topographical pattern of projections and depressions of the imprinting surface. According to the invention, high Bsat and high μ materials are preferred for use as the stamper/imprinter in order to: (1) avoid early magnetic saturation of the stamper/imprinter at the contact points between the projections of the imprinting surface and the magnetic recording layer, and (2) provide an easy path for the magnetic flux lines which enter and/or exit at the side edges of the projections.
Stampers/imprinters for use in a typical application, e.g., servo pattern formation in the recording layer of a disk-shaped, thin film, longitudinal or perpendicular magnetic recording medium comprise an imprinting surface having topographical features consisting of larger area data zones separated by smaller areas with well-defined patterns of projections and depressions corresponding to conventionally configured servo sectors, as for example, disclosed in U.S. Pat. Nos. 5,991,104 and 6,139,936, the entire disclosures of which are incorporated herein by reference. For example, a suitable topography for forming the servo sectors may comprise a plurality of projections (alt. depressions) having a height (alt. depth) in the range from about 100 to about 500 nm, a width in the range from about 50 to about 500 nm, and a spacing in the range from about 50 to about 500 nm.
According to conventional methodology, stampers/imprinters suitable for use in performing the foregoing patterning processes are manufactured by a sequence of steps as schematically illustrated in cross-sectional view in FIG. 4, which steps include providing a “master” comprised of a substantially rigid substrate with a patterned layer of a resist material thereon, the pattern comprising a plurality of projections and depressions corresponding (in positive or negative image form, as necessary) to the desired pattern to be formed in the surface of the stamper/imprinter. Stampers/imprinters are made from the master by initially forming a thin, conformal layer of an electrically conductive material (e.g., NiV) over the patterned resist layer and then electro-forming a substantially thicker (“blanket”) magnetic layer (of the aforementioned magnetic metals and/or alloys) on the thin layer of electrically conductive material, which electro-formed blanket layer replicates the surface topography of the resist layer. Upon completion of the electro-forming process, the stamper/imprinter is separated from the master, which is then re-used for making additional stampers/imprinters.
Currently, contact printing for servo patterning of magnetic media is performed on one surface of a disk at-a-time, which practice disadvantageously imposes severe limitations on product throughput rates. Another disadvantage associated with the one surface at-a-time approach is the difficulty in performing contact printing of magnetic media such that the magnetic transition pattern formed on one media surface is a mirror image of the magnetic transition pattern formed on the other (opposite) surface. Formation of mirror image magnetic transition patterns on different media surfaces is considered desirable in order to minimize “WIRRO” (Written-In-Repeatable-Run-Out) problems in multi-head hard drive devices, such as head switching during drive operation, etc., arising from differences (“incoherency”) in pattern registration between upper and lower disk surfaces.
A further impetus for developing improved methodology for performing dual-side contact printing of magnetic media is the requirement for obtaining increased product throughput rates necessary for economic competitiveness of servo-patterned disk manufacture.
Accordingly, there exists a need for means and methodology for performing dual-sided servo patterning by contact printing which are free of the above-described drawbacks and disadvantages associated with the use of conventional single-sided patterning, which means and methodology facilitate high quality, high throughput replication of servo patterns in dual-sided magnetic and MO recording media via contact printing.
The present invention addresses and solves the above-described problems, disadvantages, and drawbacks associated with prior methodologies for servo pattern formation in dual-sided thin film magnetic recording media, while maintaining full compatibility with the requirements of automated hard disk manufacturing technology.